1. Field of the Invention
The present invention generally relates to the field of display systems, particularly to a method and optical display system for producing images and presenting the images for observation in combination with an observer's visual exterior view of an outside scene.
2. Description of the Related Art
A Head Up Display (HUD) is a means of projecting information directly into a human's visual field. The HUD was pioneered for military aviation and has since been used in other applications. HUDs are typically used in aircraft to provide pilots with information superimposed onto their forward field of view through the aircraft windshield. The information displayed may be data or symbolic images indicative of flight conditions such as the operating condition of the aircraft, environmental information or guidance information for use in directing the aircraft to its destination. These images are presented in overlying fashion on the pilot's field of view so as not to interfere with the pilot's view of the background scene.
Although HUDs are useful in a variety of applications, there are several problems with conventional HUDs, among which high cost, narrow viewing angles, mechanical constraints, and low contrast of images are of particular concern. Moreover, conventional combiners are either undesirably thick or heavy for most applications, or are thin and undesirably highly curved. The thick combiners often contain a pair of cooperative lens elements, at least one of which includes an embedded spherical surface coated with a spectrally reflecting thin film. The external surfaces of these thick combiners are flat so as to provide an undistorted view of the background scene. Thin combiners, on the other hand, typically employ a pair of spherical external surfaces, one of which carries the spectrally reflecting thin film. Thin combiners thus typically do not provide the necessary undistorted view of the background scene, especially when the combiner is thick enough to be adequately durable.
The combiner of the HUD is a critical component to provide a collimated display image to a pilot for a given Field of View (FOV) at a given design reference point. As noted, the HUD combiner components are generally large in size. The current HUD combiners are typically comprised of two optical components and with reflection holograms or dielectric coatings on some of the surfaces. They are cemented together or co-mounted in the HUD system configuration which induces complicated assembly and high cost to the HUD system.
Often, HUD applications include the use of combiners with doublet lens. The doublet combiner is composed of a convex front lens and a concave back lens. The convex surface of the convex lens is dichroic or holographically coated for reflecting the HUD format information. Since the optical power of the front lens cancels the optical power of the back lens, there is no optical effect when viewing through the combiner.
U.S. Pat. No. 5,907,416, entitled “Wide FOV Simulator Heads-Up Display With Selective Holographic Reflector Combined”, issued to M. Cheren et al, discloses a wide field-of-view simulator heads-up display that provides images to a user along an optical axis. A HUD CRT provides images of a predetermined design wavelength, while rear projection screen CRTs provide images that do not contain the predetermined design wavelength. A curved holographic combiner is mounted on the optical axis with a reflective hologram on a concave surface covered with a urethane overcoat. The convex surface has an antireflective coating. A tilted beamsplitter cooperates with a folding mirror in a relay optical system for providing a virtual image of the HUD CRT for reflection from the holographic combiner.
U.S. Pat. No. 4,968,117, entitled “Graded Index Asperhic Combiners and Display System Utilizing Same”, issued to Chern et al, discloses an optical combiner that includes a substrate with at least one aspheric surface and an diffraction-type reflective coating formed on the substrate for selectively reflecting radiation within one or more predetermined narrow band ranges of wavelengths impinging on the coating. The asphericity of the surface may be selected to compensate or balance optical aberrations. The coating is advantageously a graded-index coating, which eliminates the possible degradation of gelatin hologram coatings due to moisture. A process for applying the graded-index coating to a substrate is disclosed. The variation in the index profile of the coating can be selected to provide the capability of combiners with multiple color reflectivity responses, or which allow use of display light sources of wider spectral bandwidth, resulting in a brighter image and improved angular bandwidth. In another disclosed embodiment, the combiner is incorporated in a biocular helmet visor display resulting in improved optical performance and a simpler, lightweight structure. In another embodiment, the combiner is incorporated into a head-up display for an aircraft resulting in improved performance, lower weight, improved safety and greater look-up capability.
The design of an advanced compact low cost HUD is also dependent on advancing the state of the art of the image source. The image sources of HUDs that exist today have problems relative to volume, cost, mechanical constraints, and high ambient contrast. Typical overhead mounted HUD systems of today consist of cathode-ray tubes (CRTs), projecting onto a combiner via a series of large and complex lenses. Military HUDs are typically mounted behind the forward cockpit displays. Again, they typically consist of CRTs and a series of large lenses with a turning mirror employed in order to project onto the see-through combiner.
An example of an image source for an HUD is described in U.S. Pat. No. 5,436,763, issued to C. W. Chen et al, entitled “Wide Spectral Bandwidth Virtual Image Display Optical System”. The '763 patent discloses a vertical image display optical system that uses a CRT in combination with a relay lens group to present an image to a combiner. The relay lens group includes a number of optical elements. U.S. Pat. No. 6,236,511, issued to R. D. Brown, entitled “Beam Combining Optical Element”, discloses the use of an image source that is typically a cathode-ray tube that emits an image that includes a green component carried by multiple wavelengths of light within the range of approximately 540–560 nanometers (nm). Both of these patents illustrate the inherent bulkiness of the image sources in prior art HUD systems.
Thus, it is desirable to provide a method and optical display system for producing images and presenting the images for observation in combination with an observer's visual exterior view of an outside scene, which method and system may solve the foregoing-described problems.